The healing of fractures and bone defects involves complex interactions between cells and growth factors. Currently, recombinant BMP2, a potent inducer of bone formation, is approved for use in a number of orthopedic applications, however it can generate serious side effects likely associated with the high doses used. Combining BMP2 with other modulators of bone formation could improve its efficacy and potency in orthopedic applications. Our preliminary data indicate that PDGF blocks the effects of BMP2 in periosteal progenitor cells and prevents BMP2 induced osteogenesis. PDGF and its receptors are abundant during fracture healing, however the in vivo role of PDGF during healing, and how it interacts with BMP2 signaling, is not well defined. Our hypothesis is that PDGF is a critical regulator of BMP2 signaling during bone healing. We propose three aims: In aim 1 we will define the mechanisms by which PDGF regulates BMP2 signaling. The effects of PDGF on BMP2-induced Smad signaling will be evaluated at the level of Smad phosphorylation, luciferase reporter activity and downstream gene expression in periosteal progenitor cells in vitro. We will evaluate which downstream signaling pathways are involved in the inhibition of BMP signaling by PDGF. In vivo studies will be conducted in Aim 2 and Aim 3. We will utilize models of bone fracture and bone defects. In Aim 2 we will evaluate the effects of disrupting endogenous PDGFR? signaling using an in vivo conditional deletion approach. We propose to target deletion to mesenchymal progenitor cells, chondrocytes and osteoblast lineages during fracture healing using lineage specific inducible Cre transgenic models. Effects of conditional activation of PDGFR? signaling on fracture healing will be achieved by tissue specific expression of a constitutively active form of the PDGFR? kinase domain. In Aim 3 we will evaluate the efficacy of BMP2/PDGF combination treatment delivered locally. In addition, we will evaluate the effects of a PDGFR inhibitor delivered systemically at different stages of healing allowing for precise control of dosage and timing. Combination treatments will be tested with a dose of BMP2 that alone does not induce bridging of the defect. Our project will provide better understanding on the interactions of the PDGF and BMP2 during osteogenesis and bone healing. The potential for an increase in osteogenic repair by modulating PDGF and BMP2 signaling during bone defect healing in vivo will outline the possibility of future clinical evaluation of this treatment.